Many transmitters used in communications systems today have several functions packaged together within a system on a chip (SoC). Having several functions performed within the same SoC means that there will be several clock signals having frequencies that get coupled to, and interfere with, other circuits within the IC. These clocks may be used in phase lock loops that clock processor functions or are used for modulating signals to be transmitted, etc. Accordingly, signal contamination occurs as a consequence of various signals on the SoC die, SoC substrate package, printed circuit board upon which the SoC is mounted, etc.
FIG. 1 is a simplified block diagram of a transceiver 100 having a digital signal processor (DSP) for mitigating the effects of spurious signals. A digital core 102 communicates via a port 104 connected to external devices or memory devices (not shown) in accordance with one or more protocols or network interfaces, such High-Definition Multimedia Interface (HDMI), Universal Serial Bus (USB), Ethernet, Double Data Rate (DDR), Flash, Reduced Gigabit Media Independent Interface (RGMII), etc. The digital core 102 provides signals to be transmitted to a mitigation DSP 106. The mitigation DSP 106 attempts to reduce the effects of spurious signals that are coupled to the signal to be transmitted.
Once the mitigation DSP 106 has processed the signal to reduce spurious signals generated within the digital core 102, the signals are coupled to a digital to analog converter (DAC) 108. The DAC 108 receives the signal in digital format and outputs an analog signal that can be transmitted. The analog output from the DAC 108 is coupled to a filter 110 to remove any distortion created by the DAC 108 or other out of band energy. The output of the filter 110 is coupled to a power amplifier (PA) 112. The PA 112 amplifies the signal for transmission. A receive/transmit switch 114 is set to transmit mode. In transmit mode, the switch 114 couples the output of the PA 112 to a diplexer 116. The diplexer 116 routes the output of the PA 112 to the medium 118 over which the signal is to be transmitted, such as a coaxial cable or antenna.
When the switch 114 is in the receive position, signals received over the medium 118 are routed from the diplexer 116 to a low noise amplifier (LNA) 120. The output of the LNA 120 is coupled to a filter 122 that removes out of band energy. The output of the filter 122 is coupled to an analog to digital converter (ADC) 124. The output of the ADC 124 is coupled to the mitigation DSP 106. The mitigated signals is then coupled to the digital core 102, which outputs the signals to the port 104.
The transceiver 100 shown in FIG. 1 also has a receive leg 126 for receiving satellite or cable television signals.
One common technique for mitigating interference and reducing the effects of spurious signals is to using narrow-band notch filters that reject signals that have a frequency equal to that of the interfering spurious signal. Such filters are difficult to build, since they need to be very narrow and selective. Another common technique is to use an adaptive filter to cancel transmission energy that can leak into the receive path. Yet another technique is to inject an out of phase signal that has a frequency and amplitude that is equal to the spurious signal to be cancelled.
There are several techniques for generating a signal that is 180 degrees out of phase with a spurious signal to be cancelled. These techniques are complex and expensive. Therefore, there is a need for a technique for mitigating the effects of spurious signals that can interfere with transmission signals generated within an SoC.